The present invention is directed to solid-phase peptide synthesis; more specifically, it relates to agitation of batches of reaction solutions mixed in amino-acid coupling stages with an insoluble support matrix anchoring growing peptide chains in a solid-phase synthesizing apparatus.
Applications in bioscience for solid-phase peptide synthesis are numerous and include the ready evaluation of peptide synthetic chemistry and of reaction conditions; the study of epitopes, agonists, antagonists or more potent structures; the study of structure-activity relationships; the screening and/or search of peptides to ascertain sequences; as well as the synthetic manufacture of neuropeptides, hormones and antigens.
The product at each stage of synthesis is bound to the insoluble support, and thus can be rapidly filtered and washed. The peptide chain in formation remains anchored to the support in a single vessel in which the peptide chain assembly is carried out, followed by cleavage for purification. This eliminates losses which would otherwise occur owing to the necessity of transferring products in advancing from one stage to the next.
High-efficiency synthesis is desirable, in which highly homogeneous target materials are produced through rapid assembly, reaction time for coupling the amino acids to the growing peptide chain is expedited to suppress the formation of undesired byproducts, and yield approaching quantitative mass recovery is afforded.
In a batch-wise peptide synthesizing procedure, ongoing mixing of synthesizing reagents to uniformly disperse the support material within solution batches shortens acylation time and increases efficiency by promoting rapid synthesis, which is thus advantageous for the suppression of byproducts. Alternatively to "continuous-flow synthesis," coupling reaction in batch-wise synthesis is not promoted by means of the recirculation of reagents through the peptide synthesizer reaction vessels, but rather by mixing agitation of synthesizing reagents and washing solvents remaining therein until drained or purged from the vessels.
Batch-wise peptide synthesis involving vortex mixing as a means of promoting peptide coupling is highly effective for mixing during deprotection, coupling and/or washing stages of the synthesizing process. However, mass recovery is poor, because peptidyl resin, i.e. the support material bound with elongating, protected peptide chains, is destroyed by the repetitive vortex mixing, and a portion of the peptidyl resin can then pass through the filter of the reaction vessel. Supersonic or mechanical mixing can also be employed; however these methods also tend to break or to deteriorate the peptidyl resin support material.
Peptide synthesis in which mixing is alternatively effected via bubbling inert gas through the reaction vessel (N.sub.2 gas has been used) has been successful for the production of large peptides.
The cycle of batch-wise synthesizing reactions is automated employing a simultaneous, multiple peptide synthesizer recently developed. Peptides can be produced simultaneously in channels of the synthesizer.
The reaction vessel of the automated solid-phase peptide synthesizer has a supply opening, through which batches of reaction solutions are supplied into containment by a reaction chamber defined by the reaction vessel. The chamber contains a suitable support matrix, typically particulate resin, to which the peptide chains in formation are anchored during assembly in peptide synthesis. Both between and following successive N .alpha.-deprotecting and coupling steps of the peptide synthesizing assembly procedure, a washing process is carried out, through which excess reagents and undesired products are purged via washing solvents through a drainage port of the vessel. A filter covers the drainage port and sustains the support. The drainage port at the same time serves as an inlet through which inert gas is forcibly introduced into the reaction chamber, in order to effect bubbling agitation.
During the deprotecting and coupling steps, the processes are promoted by the inert gas vigorously bubbled through the reaction chamber of the reaction vessel (the stronger the agitation, the more efficiently are the reagents likely to react with the growing peptide chains). During the subsequent washing process, the bubbling agitation improves washing efficiency as well. The support material and reagents, and the washing solution therein are, however, liable to splash out from the reaction chamber.
During peptide chain assembly, flocculation leading to clumping and coagulative skinning of the protected peptidyl resin can arise, due perhaps to interaction, among the peptide chains, of side chain aromatic rings with their protecting groups; to steric hindrance of the deprotected terminal ends of the coupling amino acids; and/or to hydrogen-bonding. This tendency becomes especially pronounced wherein peptide chains ten to fifteen amino acids or longer are synthesized, despite the action of the inert gas bubbling as a mixing agency. Moreover, the bubbling agitation, as it is increasingly vigorous, levitates the floc within the reaction chamber, such that clumped or skinned peptidyl resin is eventually compelled to break without it. This phenomenon impairs the consistency of the coupling reaction, and lowers the peptide yields.
Simply reducing the intensity of the inert gas bubbling will curb splashing of the reaction solutions and flocculation of the resin binding protected peptides. Agitation not vigorous enough can impair the coupling processes rather than promote their efficiency, however, since with low-level bubbling requiring correspondingly longer reaction time, reactions do not progress completely, defective peptides are produced and sub-reactions occur, increasing the risk that undesired by-products or deletion peptides may form, and defeating the product homogeneity and reaction efficiency sought therein.
Another alternative might be to cap the reaction vessel, but such a cap would have to be perforated in order to allow the bubbling gas to escape, which perforations would again permit the extra-vessel escape of reaction solutions or resin-particulate anchored peptides, as carried by the bubbles. Moreover, such a cap would not be effective in inhibiting or breaking the effects of the above-described flocculation, nor in bringing about more efficient mixing of the reaction solutions bathing the support.
The reagents and washing solvents of each batch of reaction solutions during a peptide synthesizing procedure are supplied through the feed tip of a tube inserted into the reaction vessel supply opening, flush with the reaction chamber so as to obviate retention of the synthesizing reagents due to capillarity at the feed tip following supply. It is desirable, however, that when washing solvent is supplied, it is distributed evenly along the cylindrical surface of the reaction chamber, in order to wash it completely of reaction solution.